Battery containing ni-based lithium transition metal oxide

ABSTRACT

The present invention provides for lithium ion secondary batteries that use Ni-based lithium transition metal oxide cathode active materials. The cathode active materials are substantially free of Li 2 CO 3  impurity and soluble bases.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/104,734, filed on Apr. 13, 2005, the disclosure of which isincorporated herein by reference.

FIELD OF INVENTION

The present invention relates to a powderous Ni-based lithium transitionmetal oxide, substantially free of soluble bases, prepared on a largescale by a low-cost process. More specifically, for preparation of theNi-based lithium transition metal oxide, inexpensive precursors,particularly Li₂CO₃ as a source of lithium, are employed, and thereaction is performed in air. The Ni-based lithium transition metaloxide is free of Li₂CO₃ impurity and has a low content of soluble basesand improved stability in air. The Ni-based lithium transition metaloxide powder can be preferably used as a cathode active material inrechargeable lithium batteries. Batteries containing such cathode activematerial exhibit high capacity, high cycling stability, much improvedstability during high temperature storage, and in particular, reducedgas evolution and improved safety.

BACKGROUND OF THE INVENTION

LiNiO₂-based cathode active materials are promising candidates toreplace LiCoO₂ in commercial rechargeable batteries. The advantages ofsuch an active cathode are summarized in the below.

(1) Price and Availability of Raw Materials:

Increasing quantities of the world production of Co are used for theproduction of LiCoO₂. This share will further increase as the actualgrowth of the Li-battery market and particularly the trend ofimplementing larger Li-batteries continues. Since Co resources arelimited, its price is expected to rise. On the other hand, the price ofNi is low, and its much larger market is expected to be able to easilyadsorb demand from a growing battery industry.

(2) Capacity:

The reversible capacity of doped LiNiO₂ is approx. 200 mAh/g whencharged to 4.3V, exceeding the capacity of LiCoO₂ (approx. 165 mAh/g).Therefore, despite a slightly lower average discharge voltage andslightly lower volumetric density, commercial cells with LiNiO₂ cathodehave an improved energy density.

However, there are severe problems that hinder the wide and successfulimplementation of LiNiO₂-based cathode active materials as described inbelow.

(A) Price:

It is generally accepted that LiNiO₂ of high quality cannot be preparedby such simple methods as are used for LiCoO₂ production, i.e., simplesolid state reaction of a Co precursor with LiCoO₂. Actually, dopedLiNiO₂ cathode materials in which an essential dopant is cobalt andfurther dopants are Mn, Al, etc. are produced on a large scale byreacting lithium precursors such as LiOH*H₂O with mixed transition metalhydroxides in a flow of oxygen or synthetic air (i.e., CO₂ free). Also,additional steps such as an intermediary washing or coating furtherincrease the cost of such processes.

(B) Safety, Gassing, Gelation and Aging:

-   -   Safety: the implementation of LiNiO₂ has been delayed by        concerns about the safety of LiNiO₂ batteries. The safety of the        cathode powder can be increased to some extent, for example, by        modifying the composition of the cathode powder or optimizing        the morphology. Furthermore, the safety of batteries can be        improved by battery design, electrolyte modifications, etc.    -   Storage properties: the commercial implementation of LiNiO₂ has        particularly been delayed due to poor storage and abuse        properties. A severe problem, which has not been solved yet, is        the evolution of an excessive amount of gas during storage or        cycling. Excessive gas activates the safety switch to shut down        a cylindrical cell and also causes a polymer battery to swell.        The inventors of the present invention found that there is a        correlation between the content of soluble base and the        excessive gas evolution, and particularly that the amount of        Li₂CO₃ (as determined by pH titration) has a close relation to        the amount of gas evolved during storage.    -   Processing: another problem of LiNiO₂ involves the stability of        the cathode material (when exposed to air and humidity, LiNiO₂        deteriorates rapidly) and the gelation of slurries (due to a        high pH, the NMP-PVDF slurry starts to polymerize). These        properties cause severe processing problems during battery        production.

Many prior arts focus on improving properties of LiNiO₂-based cathodematerials and processes to prepare LiNiO₂. However, the problems of highproduction cost, swelling, poor safety, high pH and the like have notbeen sufficiently solved. A few examples will be illustrated in below.

U.S. Pat. No. 6,040,090 (T. Sunagawa et al., Sanyo) discloses a widerange of compositions including nickel-based and high-Ni LiMO₂, thematerials having high crystallinity and to be used in Li-ion batteriesin EC containing electrolyte. Samples were prepared on small scale,using LiOH*H₂O as a lithium source. The samples are prepared in a flowof synthetic air being a mixture of oxygen and nitrogen, free of CO₂.

U.S. Pat. No. 5,264,201 (J. R. Dahn et al.) discloses a doped LiNiO₂substantially free of lithium hydroxide and lithium carbonate. For thispurpose, transition metal hydroxide and LiOH*H₂O as a lithium source areemployed and heat treatment is performed under an oxygen atmosphere freeof CO₂, additionally with a low content of H₂O. An excess of lithium“evaporates”; however, “evaporation” is a lab-scale effect and not anoption for large-scale preparation.

U.S. Pat. No. 5,370,948 (M. Hasegawa et al., Matsushita) discloses aprocess to prepare LiNi_(1−x)Mn_(x)O₂ doped by Mn, x<0.45, in which themanganese source is Mn-nitrate, and the lithium source is either lithiumhydroxide or lithium nitrate.

U.S. Pat. No. 5,393,622 (Y. Nitta et al., Matsushita) discloses aprocess to prepare LiNi_(1−x)Mn_(x)O₂ by a two-step heating, involvingpre-drying, cooking and the final heating. The final heating is done inan oxidizing gas such as air or oxygen. This patent focuses on oxygen.The disclosed method uses a very low temperature of 550˜650° C. forcooking, and less than 800° C. for sintering. At higher temperatures,samples are dramatically deteriorated. Excess lithium is used such thatthe final samples contain a large amount of soluble bases (i.e., lithiumcompounds). According to research performed by the inventors of thepresent invention, the observed deterioration is attributable to thepresence of lithium salts and melting at about 700˜800° C., therebydetaching the crystallites.

WO 9940029 A1 (M. Benz et al., H. C. Stack) describes a complicatedpreparation method very different from that disclosed in the presentinvention. This preparation method involves the use of lithium-nitratesand lithium hydroxides and recovering the evolved noxious gasses.Sintering temperature never exceeds 800° C. and typically is far lower.

U.S. Pat. No. 4,980,080 (Lecerf, SAFT) describes a process to prepareLiNiO₂-based cathodes from lithium hydroxide and metal oxides attemperatures below 800° C.

In prior arts including the above, LiNiO₂-based cathode active materialsare generally prepared by high cost processes, especially in a flow ofsynthetic gas such as oxygen or synthetic air, free of CO₂, and usingLiOH*H₂O, Li-nitrate, Li acetate, etc. but not the inexpensive, easilymanageable Li₂CO₃. Furthermore, the final cathode materials have a highcontent of soluble bases, originating from carbonate impurities presentin the precursors, which remain in the final cathode because of thethermodynamic limitation. To remove the soluble bases, additional stepssuch as washing, coating etc. are required, thereby increasing the cost.

Therefore, there is a strong need for LiNiO₂-based cathode activematerials able to be prepared at low cost from inexpensive precursorssuch as Li₂CO₃, having a low content of soluble base, showing improvedproperties such as low swelling when applied to commercial rechargeablelithium batteries, improved safety and high capacity.

SUMMARY OF THE INVENTION

The objects of the present invention are to completely solve theproblems described above.

In accordance with the present invention, the above and other objectscan be accomplished by the provision of a powderous lithium transitionmetal oxide with the composition as represented by Formula 1 below, withbeing practically free of Li₂CO₃ impurity, and prepared by solid statereaction in air from a mixed transition metal precursor and Li₂CO₃:

Li_(x)M_(y)O₂  (1)

wherein

M=M′_(1−k)A_(k), where

-   -   M′=Ni_(1−a−b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b) on condition of        0.65≦a+b≦0.85 and 0.1≦b≦0.4;    -   A is a dopant;    -   0≦k<0.05; and

x+y=2 on condition of 0.95≦x≦1.05.

As defined above, the powderous lithium transition metal oxide consistsof nickel, manganese and cobalt at a specific composition and has a highcontent of nickel, and also optionally may further contain less than 5%of dopant (A).

The Ni-based lithium transition metal oxide according to the presentinvention has a well-layered structure, and also improved safety,cycling stability and stability against aging and low gas evolutionduring storage, when used as an active material for cathode of lithiumsecondary batteries, because it has a high sintering stability and issubstantially free of soluble bases.

Moreover, the lithium transition metal oxide of the present inventioncan be prepared by a low-cost process under relatively unrestrictedconditions using a mixed transition metal precursor and Li₂CO₃ as rawstocks.

In a process for preparation of the lithium transition metal oxide,Li₂CO₃ of a low cost is employed as a lithium source, and lithium is notused in an excess amount, and heat treatment is carried out under highflow of air in a reactor, preferably equipped with a heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a graph showing the preferred composition range of Ni-basedlithium transition metal oxide according to the present invention;

FIG. 2 is a graph showing the pH titration of soluble bases fromcommercial cathode materials in Comparative Example 1;

FIG. 3 is a graph showing the standard pH titration of soluble basesfrom LiOH*H2O and Li2CO3;

FIG. 4 is FESEM micrographs of commercial high-Ni LiNiO2 in ComparativeExample 2 in which (A) is the FESEM of sample as received and (B) is theFESEM of sample after heating to 850° C. in air;

FIG. 5 is a graph showing the standard pH titration of commercialhigh-Ni LiNiO2 in Comparative Example 3 in which (A) is for the sampleas received, (B) is for the sample after heating to 800° C. in oxygenatmosphere, and (C) is for a control group;

FIG. 6 is a graph showing the decomposition rate of commercial high-NiLiNiO2 during air storage in a 90% humidity chamber at 60° C. by pHtitration in Comparative Example 4 in which (A) is for the sample asreceived, (B) is for the sample after 17-hours storage in the humiditychamber, (C) is for the sample after 3 days storage in the humiditychamber;

FIG. 7 is a graph showing the DCS measurements, of the samples fromComparative Example 6 in which (A) is for the commercial Al/Ba-modifiedLiNiO2 and (B) is for the commercial AlPO4-coated LiNiO2; where the DCScombustion test is a measure of the safety of the specimen and itsstability to storage.

FIG. 8 is FESEM micrographs (×2000) of the sintered nickel-based LiMO2of Example 1: A) 850° C., (B) 900° C., (C) 950° C. (D) 1000° C.;

FIG. 9 is a crystallographic map of the samples with different Li:Mratios in Example 2;

FIG. 10 shows the pH titration of the samples with different Li:M ratioin Example 2;

FIG. 11 is SEM micrographs of the cathode active material in Example 3;

FIG. 12 shows the Rietveld refinement of the X-ray diffraction patternof the sample in Example 3;

FIG. 13 is graphs showing the electrochemical properties of nickel-basedLiMO2 prepared in air using Li2CO3 in Example 5 in which (A) is a graphshowing the voltage profile and rate performance at room temperature(cycle 1-7), (B) is a graph showing the cycling stability (3.0-4.3V) atC/5 rate at 25° C. and at 60° C., (C) is a graph showing the dischargeprofile (C/10 rate) of cycle 2 and cycle 31 obtained during 25° C. and60° C. cycling;

FIG. 14 shows the result of DCS safety testing of the high-Ni LiNiO2 inExample 6;

FIG. 15 shows the results of electrophysical properties tests on thepolymer cell in Example 7;

FIG. 16 is a graph showing the swelling of polymer cell during high Tstorage in Example 7;

FIG. 17 is a graph showing the air stability of large-scale samplemeasured by pH titration in Example 8 in which (A) is for a freshsample, (B) is for the sample after 17 h storage, and (C) is for thesample after 3d storage;

FIG. 18 is SEM micrographs (×5000) of the precursor and the finalcathode material in Example 10 in which (A) is for a precursor preparedby an inexpensive ammonia-free process and having a low density, and (B)is for LiMO2 prepared in air using Li2CO3 as a precursor.

DETAILED DESCRIPTION

The present invention will be described in more detail.

Stoichiometric LiNiO₂ in which the transition metal consists of only Niand the Li:Ni ratio is 1:1 essentially does not exist or is extremelydifficult to prepare. Instead, Li-deficient Li_(1−a)Ni_(1+a)O₂ and dopedLiNi_(1−z)M″_(z)O₂ (M″=Co, Mn_(1/2)Ni_(1/2), Al . . . ) with a Li:Mratio of 1:1 are more easily achieved.

In the present invention, only the doped LiNiO₂ is handled and, forconvenience of expression, sometimes referred to as “LiNi_(1−z)M″,O₂” or“doped LiNiO₂” in the present disclosure. Generally, the doped LiNiO₂may be in the stoichiometric form or Li-deficient form. Therefore, thestoichiometric form (Li:M=1:1) and Li-deficient form (Li:M<1:1) in thepresent disclosure will be sometimes referred to as “stoichiometricLiNiO₂” and “Li_(1−a)Ni_(1+a)O₂”, respectively. The doped LiNiO₂ has alower content of 3-valent nickel than pure LiNiO₂ but a higher contentthan any doped LiCoO₂ or LiMnO₂. In the present disclosure, the term“high-Ni LiNiO₂” means that ‘z’ in the formula LiNi_(1−z)M″,O₂ is 0.7 ormore.

The stoichiometric LiNiO₂ (i.e., LiNi_(1−z)M″_(z)O₂, but not beingLi-deficient) is desirable because it shows a superior electrochemicalperformance. Li-deficient samples have cation mixing. Cation-mixedsamples have transition metal cations being misplaced on lithium sitesof the crystal structure. Lithium-deficient Li-Ni-oxide is undesiredbecause it has higher cation mixing which causes poor electrochemicalproperties.

According to the present invention, the composition of the lithiumtransition metal oxide must satisfy the specific conditions as definedin Formula 1 above, which can be expressed as the below or in FIG. 1.

Ni_(1−a−b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b) and 0.65≦a+b≦0.85  (i)

0.1≦b≦0.4  (ii)

x+y=2 and 0.95≦x≦1.05  (iii)

With respect to the condition (i), where the content of 3-valent nickelis excessively high, i.e., a+b<0.65, the doped LiNiO₂ cannot be preparedin air on a large scale and Li₂CO₃ cannot be used as a precursor (seeComparative Example 2). On the other hand, where the content of 3-valentnickel is excessively low, i.e., a+b>0.85, the doped LiNiO₂ can beprepared in air on a large scale and Li₂CO₃ can be used as a precursor;however, the volumetric capacity of the doped LiNiO₂ thus prepared isnot competitive compared to that of LiCoO₂ (see Comparative Example 8).

With respect to the condition (ii), where the content of cobalt isexcessively high, i.e., b>4.5, the overall cost of raw materialsincreases because of a high content of cobalt and a slightly lowerreversible capacity results. On the other hand, where the content ofcobalt is excessively low (b<0.1), it is substantially difficult toachieve a sufficient rate performance and high powder density ofbatteries at the same time.

With respect to the condition (iii), where the content of lithium isexcessively high, i.e., x>1.05, a poor stability is exhibited duringcycling at a high voltage (U=4.35 V), particularly at T=60° C. On theother hand, where the content of lithium is excessively low, i.e.,x<0.95, a poor rate performance is exhibited and accordingly thereversible capacity is reduced.

As mentioned previously, the lithium transition metal oxide may furthercontain dopant in a minor amount. Typical dopants are Al, Ti and Mgwhich are incorporated into the crystal structure. The low doping levelsof these dopants (<5%) may be helpful in increasing the general safetyand storage and overcharge stability of batteries without significantlowering of reversible capacity. Other dopants known in the art, such asB, Ca, Zr, S, F, P, Bi etc., are not incorporated into the crystalstructure but are accumulated at grain boundaries or coat the surfacethereof. However, small concentrations of such dopants (<1%) mightenhance the stability without lowering the reversible capacity whenapplied at very low doping levels (<1%). Therefore, various dopants asdescribed above can be applied to the present invention.

The lithium transition metal oxide of the present invention is preparedby solid state reaction in air by an inexpensive process.

The solid state reaction in air proceeds preferably through a two-stepheating procedure comprising: (i) a cooking step at a temperature ofbetween 700 and 950° C. under air circulation and then (ii) a sinteringstep at a temperature of between 850° C. and 1020° C.

As raw materials for the solid state reaction, lithium carbonate(Li₂CO₃) and a mixed transition metal precursor are used. Li₂CO₃ servesas a source of lithium. The mixed transition metal precursor includes,for example, but is not limited to mixed hydroxides, mixed carbonatesand mixed oxides. Herein, “mixed” means that several transition metalelements are well mixed at the atomic level.

One of the features of the present invention is that inexpensive rawmaterials or materials produced by an economical process can be used,and also Li₂CO₃ which is difficult to employ in the prior art is useditself.

MOOH (M=Ni, Mn and Co), as a representative example of the mixedtransition metal precursor, has been prepared in the prior art byco-precipitation of MSO₄ and NaOH in the presence of excess ammonia as acomplexing additive to obtain. MOOH having a high density. However,ammonia in waste water causes environmental problems and thus isstrictly regulated. On the other hand, MOOH of a relatively low density,which is prepared by a less inexpensive process without using ammonia(‘ammonia-free process’), can be employed if the doped LiNiO₂ producedtherefrom can tolerate stronger sintering conditions (see Example 1).

In conventional processes, Li₂CO₃ cannot be used as a raw stock becausethe decomposition of Li₂CO₃. for production of LiMO₂ would generate CO₂which decomposes the high-Ni LiNiO₂. Moreover, this side reaction occurseven when Li₂CO₃ is present as an impurity in precursors of theresulting LiNiO₂. On the other hand, in the present invention, thesephenomena are not caused in any case where Li₂CO₃ is used as a raw stockor contained in precursors. Furthermore, the doped LiNiO₂ of the presentinvention is substantially free of Li₂CO₃.

In the present disclosure, pH titration is widely used to find orconfirm many experimental results, including the above result. pHtitration was performed, for example, in the following manner: 5 g ofcathode powder is immersed (soaked) into 25 ml water, and after briefstirring, about 20 ml of clear solution is separated from the powder bydecanting, then the clear solution is collected. Again, about 20 mlwater is added to the powder, stirred, and collected after decanting.The soaking and decanting is repeated at least 3 more times. By thismanner, a total of 100 ml clear solution is collected which containssoluble bases. The content of soluble base is measured by pH titration.While stirring, a flow of 0.1M HCl is added to the solution, and pH as afunction of time is recorded. The experiment is finished when the pHreaches a value below pH=3. The flow rate is chosen so that thetitration takes about 20˜30 minutes. The content of soluble base isgiven by the amount of acid used to reach pH below 5. The content ofsoluble base obtained for a given powder in this manner is reproducible,but does depend very weakly on other parameters such as the totalsoaking time of powder in water. Bases are contributed mainly from twosources: first, impurities such as Li₂CO₃ and LiOH present in theLiNiO₂; second, base originating from ion exchange at the surface ofLiNiO₂ (H⁺ (water)← →Li⁺ (surface, outer bulk). The second contributionis typically negligible.

As mentioned previously, the lithium transition metal oxide of thepresent invention contains substantially no Li₂CO₃ impurity and containsonly a low content of soluble bases. The level of soluble base contentis such that, for example, less than 20 ml of 0.1M HCl is needed totitrate 200 ml of solution to achieve pH below 5, in which the 200 mlsolution contains substantially all soluble bases and also is preparedby repeated soaking and decanting of 10 g of the lithium transitionmetal oxide. It is more preferably less than 10 ml of 0.1M HCl.

In addition, variations due to scale occur in preparation of the dopedLiNiO₂. Samples of a few grams in a furnace behave very differently fromsamples of a few kg, because the gas transport kinetics at low partialpressure is very different. Especially in a small-scale process, Lievaporation occurs and CO₂ transport is fast, whereas in a large-scaleprocess, these processes are retarded. In this connection, it is notedthat the term “large scale” in the present disclosure means a samplesize of 5 kg or more because similar behavior is expected in 100 kg ofsample when the process has been correctly scaled-up, i.e., a similargas flow (m³/kg of sample) reaches the sample of 100 kg.

The lithium transition metal oxide of the present invention can beproduced preferably through a large-scale process, which is veryimportant in view of practical application. For the solid state reactionin air as mentioned above, air is pumped into or out of a reactor toachieve rapid air circulation in which at least 2 m³ of air (volume atroom temperature), preferably at least 10 m³ of air per 1 kg of thefinal lithium transition metal circulates through the reactor during thereaction.

In an embodiment of the present invention, a heat exchanger is used topre-warm the in-flowing air before it enters the reactor, while coolingthe out-flowing-air.

In a preferable embodiment, the solid state reaction occurs by at leasttwo steps, including a cooking step at a temperature between 700 and950° C. where the transition metal precursor and the Li₂CO₃ react toform a LiMO₂ precursor, and a sintering step at a temperature between850 and 1020° C. where the final LiMO₂ with a well-layered crystalstructure is achieved, in which during the cooking step large quantitiesof air exceeding 2 m³/kg LiMO₂ is fed into the reactor equipped with aheat exchanger to preheat the air.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Now, the present invention will be described in more detail withreference to the following. Examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Comparative Example 1 pH Titration of Li₂CO₃ Impurity in CommercialCathode Materials

pH titration was performed for two batches “A” and “B” of the samecommercial cathode active materials supplied by the same producer. Thecomposition of cathode materials was given as Li_(1.05)M_(0.95)O₂ withM=(Mn_(1/2)Ni_(1/2))_(0.83)Co_(0.17). The cathode material was appliedto pilot plant cells. During the high-temperature storage of thesecells, cells containing batch “A” evolved unacceptable amounts of gaswhereas cells containing batch “B” did not it. Besides this, the batcheswere identical or very similar in all investigated aspects such asmorphology, BET surface area, crystallite size, particle size,reversible capacity, rate performance, crystal structure, latticeparameters, cation mixing, etc.

However, the results of pH titration were very different. Forcomparison, the pH profile of a commercial sample “C” with thecomposition of LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ was also measured becausepilot plant cells containing this sample showed an exceptionally low gasevolution. The result of the pH titration experiments is provided inFIG. 2.

Referring to FIG. 2, Sample “A” showing a strong gas evolution containedan excessive quantity of soluble base. Sample “C” having an exceptionalstability was substantially free of soluble base. From the shape of thepH titration profile, the character of the soluble base can be obtained.With reference to FIG. 3, Li₂CO₃ shows two plateaus, whereas LiOH hasonly a single plateau at high pH. Therefore, the soluble base of sample“A” is identified to be mainly Li₂CO₃. Sample “B” contained a smallamount of Li₂CO₃-type base and a still smaller amount of LiOH-type base,probably originating from molecules on the surface or from the ionexchange reaction between water and lithium present in the outermostregion of cathode particles.

Knowledge about the content of soluble bases is a powerful tool to guidethe development of cathodes with improved storage stability. It is,however, important to measure the pH profile in order to characterizewhich soluble bases are present. Only measuring pH, for example, asdescribed in EP 1 317 008 A2 (S. Miasaki, Sanyo) is not recommendablebecause even a small amount of LiOH-type impurity (which is quiteharmless) can give a higher pH than that obtained for a significant andharmful Li₂CO₃ impurity.

Therefore, this experiment clearly shows the usefulness of pH titrationto obtain information about the content of soluble bases.

Comparative Example 2 Thermodynamic Stability of Commercial High-NiLiNiO₂

In this experiment, the thermodynamic stability of commercial LiNiO₂ wasinvestigated. The sample had the composition ofLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ which may be alternatively expressed asLiNi_(1−x)M_(x)O₂ with x=0.3, i.e., M=Mn_(1/3)Ni_(1/3)Co_(1/3).

The thermodynamic stability was measured by heating the above cathodematerial in air. 50 g of each sample was heated to each of 500° C. (48h), 750° C., 800° C., 850° C., 900° C. and 950° C. (36 h). X-rayanalysis was performed to obtain detailed lattice parameters with highresolution. The cation mixing was obtained by Rietveld refinement.Morphology was investigated by field emission electron microscopy(FESEM).

The X-ray analysis showed the continuous deterioration of crystalstructure (increase of cation mixing, increase of lattice constant, andlowering of c:a ratio) for all samples heated to T≧750° C. The high-NiLiNiO2 decomposes in air containing trace CO2 with reduction of 3-valentNi according to the below scheme.

LiMiiiO2+CO2→aLi1−xM1+x1iii,iiO2+b Li2CO3+cO2

In this procedure, the increase of Li2CO3 impurity was ascertained by pHtitration.

In FIG. 4, a micrograph of the commercial sample as received is comparedwith that of the same sample heated to 850° C. Referring to FIG. 4, thesample heated to T≧850° C. has disintegrated. In an additionalexperiment, a full disintegration of secondary particles into singleprimary crystallites was observed at 900° C.

In summary, the commercial LiNiO2 cathode materials arethermodynamically unstable during heating in air. More specifically,Li2CO3 forms and the molten Li2CO3 separates the grains so that primaryparticles lose contact and the secondary particles collapse.Accordingly, it is impossible to prepare Li-Ni-oxides with a high Nicontent, i.e., LiNi1−xMxO2 with x≧0.7 in air due to the thermodynamiclimitation, where the air contains trace CO2 at a sufficiently highpartial pressure. It is also ascertained in this experiment that Li2CO3cannot be used as a precursor in conventional processes, because thedecomposition of Li2CO3 for formation of LiMO2 gives CO2, which wouldkinetically hinder a further decomposition even at a low partialpressure.

Comparative Example 3 Li₂CO₃ Impurity in Commercial High-Ni LiNiO₂

In this experiment, it was investigated whether the stoichiometric andimpurity-free high-Ni LiNiO2 can be obtained on a large scale by asimple process involving solid state reaction in oxygen.

In prior art processes, as precursors for preparation of LiNiO2-basedcathode, LiOH*H2O and Ni-based transition metal hydroxide are generallyemployed. However, both precursors commonly contain carbonateimpurities. The technical grade LiOH*H2O typically contains >1% Li2CO3impurity, and Ni(OH)2 also contains CO3 anion because it is prepared byco-precipitation of a Ni-based salt such as NiSO4 with a base such asNaOH in which the technical grade NaOH contains Na2CO3 and the CO3 anionis more preferably inserted into the Ni(OH)₂ structure than the OHanion.

When cooking a mixture of these precursors in oxygen, the lithiumhydroxide and transition metal hydroxide react to form Li1−xM1+xO2, butall carbonate impurity is trapped as Li2CO3 impurity. The Li2CO3impurity does not decompose at a sufficient rate during further cookingin oxygen, and the stoichiometric high-Ni LiNiO2 is very unstable at800° C. As a result, no Li2CO3 decomposes but Li2O additionally forms,as will be described in below.

In this regard, the pH titration of commercial high-Ni LiNiO2 of whichthe composition is LiNi0.8Co0.2O2 is shown in FIG. 5. Curve (A) in thisdrawing shows the pH titration of the LiNi0.8Co0.2O2 as received, andCurve (B) after heating to 800° C. for 24 hours in a flow of pureoxygen. Curve (C) is a copy of curve (A) and allows to better displaythe similarity of the shapes of curve (A) and (B). Flow rate was >2l/min and the sample was 400 g. The analysis of the pH profile showsthat the contents of Li2CO3 before and after heat treatment areidentical. Apparently, the Li2CO3 impurity did not react at all, whereasa small amount of Li2O has formed (The corresponding slight decrease ofLi content in the LiNiO2 crystal structure was confirmed by theobservation of a slight increase of cation mixing, slight decrease ofc:a ratio and slight decrease of unit cell volume obtained from X-rayanalysis).

It can be concluded that conventional methods (heating of Ni(OH)2 andLiOH*H2O) in a “normal” flow of oxygen gas or synthetic air do notachieve stoichiometric and impurity-free LiNiO2 on a large scale.Herein, the “normal” flow means a flow of less than about 1 m3 gas fedinto the reactor per kg of cathode material during the reaction. Eitherwhen the LiNiO2 contains a significant Li2CO3 impurity, or when theLi2CO3 impurity is avoided, the LiNiO2 will necessarily become lithiumdeficient (i.e., cation mixed) Li1−xNi1+xO2. This is because theequilibrium partial pressure of CO2 for Li2CO3 coexisting withLi1−xNi1+xO2 strongly increases with “x”; therefore, the reactiontowards stoichiometric LiNiO2 is kinetically limited by the poor gastransport kinetics of CO2 at low pressure. Only if Li is sufficientlydeficient, i.e., “x” is sufficiently large, does the higher CO2equilibrium partial pressure allow for a significant transport of CO2away from the sample so that Li2CO3 (originating from CO3 anionimpurities of the precursors) effectively decomposes. Alternatively,modification of the prior art processes, for example, by pumping muchlarger flows of oxygen or synthetic air at lower cooking temperature,would increase the process cost. An intermediary washing procedure,which would effectively remove unreacted Li2CO3, followed by heattreatment, would also significantly increase the process cost.

Comparative Example 4 Air Stability of Commercial High-Ni LiNiO₂

The pH titration result of commercial high-Ni LiNiO₂ before and afterexposure to humid air is shown in FIG. 6. The commercial LiNiO₂ isLiAl_(0.02)Ni_(0.78)Co_(0.2)O₂, additionally containing less than 1 ofbarium compounds, and the results of FIG. 6 show that the amount ofsoluble base before storage is exceptionally low. It is expected thatthe producer has prepared the sample in oxygen, either from extremelypure (i.e., CO₃ anion-free) precursors or by applying at least twocooking steps, interrupted by a washing procedure to remove Li₂CO₃ andLiOH impurities. Barium is probably added to trap the remaining CO₃anions by forming the highly stable BaCO₃. This manner is a high-costprocess.

Upon air exposure, a significant amount of soluble base, mainly Li₂CO₃type, continuously forms. The result shows that commercial LiNiO₂, evenif the initial content of Li₂CO₃ impurity is low, is not stable in airand decomposes at a significant rate, and a significant amount of Li₂CO₃impurity is formed during storage.

Comparative Example 5 Air Stability of Commercial Coated High-Ni LiNiO₂

Another commercial high-Ni LiNiO₂ sample with the composition ofLiNi_(0.8)Mn_(0.05)Co_(0.15)O₂ was tested. The preparation process ofthe sample includes a surface coating by AlPO₄, followed by a mild heattreatment, and this is a high cost process. The coating is probably adip-coating process, having the side effect that excess Li₂CO₃ isdissolved. Furthermore, during the heat treatment, AlPO₄ reacts withexcess lithium so that Li₃PO₄ and Al₂O3 (or LiAlO₂) can form. Therefore,the sample has a low content of Li₂CO₃ and the surface of the cathodematerial is lithium-deficient. The experimental results confirmed areduced swelling property in polymer cells. By the pH titration result,a low initial Li₂CO₃ content (12 ml 0.1M HCl per 10 g cathode) wasascertained. The profile was very similar to that of the fresh sample ofComparative Example 4, Curve (A). Two more pH profiles were recordedafter storage in a humidity chamber similar to Comparative Example 4.Only a slightly lower formation rate of Li₂CO₃ (80˜90%) compared withComparative Example 4 was observed.

These results show that the coating of high-Ni LiNiO₂ does not improveits stability during storage in air. Furthermore, electrochemicalproperties such as the cycling stability and rate performance were poor,which was possibly caused by the lithium-deficient surface.

Comparative Example 6 Safety of Commercial High-Ni LiNiO₂

The result of DSC measurement is shown in FIG. 7. For the measurement,coin cells (Li metal anode) with LiNiO₂ cathodes were charged to 4.3 V,and after disassembly they were inserted into hermetically sealed DSCcans, and electrolyte was poured thereinto. The total amount of cathodewas about 50˜60 mg and the amount of electrolyte was approximately thesame. As such, the exothermic reaction is strongly cathode-limited (onlya fraction of the electrolyte can be fully combusted by all oxygen ofthe cathode). The DSC measurement was performed at a heat rate of 0.5K/min.

Referring to FIG. 7, in both (A) Al/Ba-modified LiNiO₂ and (B)AlPO₄-coated LiNiO2, a strong exothermic reaction starts at relativelylow temperatures. In the case (A), the heat evolution exceeds the limitof the device. The total integrated amount of evolved heat is large,well above 2000 kJ/g, indicating the poor safety of commercial high-NiLiNiO₂.

Although further attempts to improve the performance of high-Ni LiNiO₂are disclosed in many prior art literatures and patents, these methodsare expensive and the results are usually insufficient. Alternatively,encapsulation of high Ni-LiNiO₂ by SiO_(x) protective coating has beenproposed (H. Omanda, T. Brousse, C. Marhic, and D. M. Schleich, J.Electrochem. Soc. 151, A922, 2004), but the resulting electrochemicalproperties are very poor. In this connection, the inventors of thepresent invention have investigated the encapsulation by LiPO₃ glass.Even where a complete coverage of the particle is accomplished, asignificant improvement of air-stability could not be made andelectrochemical properties were poor.

Comparative Example 7 Electrochemical Properties of Commercial High-NiLiNiO₂

In Table 1 below, the results of electrochemical testing of differentcommercial high-Ni LiNiO₂ materials are summarized. The testing wasperformed at 60° C. at C/5 charge and discharge rate. The charge voltagewas 4.3 V. Referring to Table 1, with the exception of Sample (B), thecycling stability is poor. The poor cycling stability of Sample (C) isprobably caused by the Li-deficiency of the surface (the poor capacityretention of cation-mixed (i.e., Li-deficient) lithium nickel oxides isknown in the prior art literatures). Both Samples (A) and (B) arestoichiometric (i.e., not Li-deficient), but only Sample (B) has a lowcontent of Li₂CO₃. The presence of Li₂CO₃ may not only cause gassing butalso fading (Probably at 4.3 V, Li₂CO₃ slowly decomposes and thecrystallites lose electrical contact).

Therefore, the problems of safety, poor air stability, high Li₂CO₃impurity level and high processing cost have not been solved in theprior art processes.

TABLE 1 Electrochemical properties of high Ni—LiNiO₂ (60° C., C/5-C/5,3.0-4.3 V) (A) (B) Al/Ba- LiNi_(0.8)Co_(0.2)O₂ modified (C) AlPO₄-coatedDescribed in Comp. Ex. 3 Comp. Ex. 4 Comp. Ex. 5 StoichiometryStoichiometric Stoichiometric Surface Li Li:M high low deficient Li₂CO₃low impurity Capacity at 193, 175 mAh/g 195, 175 mAh/g 185, 155 mAh/g25° C. C/10, C/1 Capacity loss 30% per 100 11% per 100 >30% per 100cycles cycles cycles

Comparative Example 8 Volumetric Capacity of Commercial Low-Ni LiNiO₂

Commercial LiMO₂ with M=(Ni_(1/2)Mn_(1/2))_(1−x)Co_(x) with x=0.17 andwith x=0.33, respectively, were tested. The crystallographic densitiesthereof were approx. 4.7 and 4.76 g/cm³, respectively. A dischargecapacity of 157˜159 mAh/g at C/10 rate (3˜4.3 V) was obtained for bothmaterials.

The crystallographic density of LiCoO₂ is 5.04 g/cm³ and the dischargecapacity is 157 mAh/g. The volumetric capacity of the cathode withx=0.17 corresponds to only 93% of the density of LiCoO₂ and the densityof the cathode with x=0.33 corresponds to only 94%. Accordingly, it isascertained that materials with low Ni content have a poor volumetriccapacity.

Example 1 Sintering Stability

A mixed hydroxide MOOH with M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)was used as the transition metal precursors for preparation ofnickel-based LiMO₂. The transition metal composition of the final LiMO₂is marked as an asterik in FIG. 1. An intermediary sample was preparedby mixing the mixed hydroxide and Li₂CO₃ (stoichiometric ratioLi:M=1.02:1) and heating the resulting mixture to 700° C. in air.Samples (each approx. 50 g) of the intermediary sample were thensintered for 15 hours at various temperatures from 700 to 1000° C. innormal air. An improved sintering stability was observed at all of thetemperatures. Secondary particles were maintained intact, and nodisintegration into single crystallites as observed in ComparativeExample 2 was observed. The size of crystallites increased with thesintering temperature.

X-ray analysis showed that all samples have a well-layered crystalstructure. The unit cell volume did not change significantly withincrease of sintering temperature, which proves that no significantoxygen deficiency, no significant increase of cation mixing andessentially no Li evaporation occurred. The best electrochemicalproperties were obtained in samples sintered around 900° C., having apreferred BET surface area of about 0.4˜0.8 m²/g. The obtainedcrystallographic data are provided in Table 2 below, and FESEMmicrographs in FIG. 8.

This experiment shows that despite the use of Li₂CO₃ and sintering inair, well-layered, stoichiometric LiMO₂ can be made, and furthermore anexcellent sintering stability in air can be obtained.

TABLE 2 Crystallographic data Sintering temperature (A) 850° C. (B) 900°C. (C) 950° C. (D) 1000° C. Unit cell 33.902 Å³ 33.905 Å³ 33.934 Å³33.957 Å³ volume Normalized c:a 1.0123 1.0124 1.0120 1.0117 ratio c:a/24{circumflex over ( )}0.5 Cation mixing 4.5% 3.9% 4.3% 4.5% fromRietveld refinement

Comparative Example 9 Sintering Stability of High Co Samples

Mixed hydroxide MOOH with M=Ni_(0.25)(Mn_(1/2)Ni_(1/2))_(1/3)Co_(5/12)was used as a precursor. The amount of 3-valent Ni in LiMO₂ was almostidentical to that in Example 1. The same investigation as in Example 1was performed. Samples could be sintered in air with basically nodisintegration being observed. Crystallographic data are summarized inTable 3 below. Apparently, the sintering stability is rather similar tothat of LiNiO₂ (M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2) produced inExample 1. It was expected that the high Co content would hinder cationmixing; however, the cation mixing was surprisingly similar or greater.

Unfortunately, high cobalt-content samples are more expensive due tohigh prices of cobalt as a raw material. Moreover, the additionalelectrochemical test showed slightly lower reversible capacities incomparison with that of Example 1. Therefore, the composition range, notbeing too rich in cobalt, as sketched in FIG. 1, is a preferable regionfor the present invention.

TABLE 3 Crystallographic data Sintering temperature (A) 900° C. (B) 950°C. (C) 1000° C. Unit cell volume 33.445 Å³ 33.457 Å³ 33.514 Å³Normalized c:a 1.0144 1.0142 1.0154 ratio c:a /24{circumflex over( )}0.5 Cation mixing from 3.3% 6.3% 6.6% Rietveld refinement

Example 2 Li Stoichiometric Range

Samples with different Li:M ratios were prepared from MOOH withM=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2). Li₂CO₃ was used as alithium source. 7 samples each of about 50 g with Li:M ratios rangingfrom 0.925 to 1.12 were prepared through two steps. The samples werefirst cooked at 700° C., followed by sintering at 910˜920° C. All heattreatments were carried out in normal air. Then, electrochemicalproperties were tested.

Table 4 below provides the obtained crystallographic data. The unit cellvolume changes smoothly according to the Li:M ratio. FIG. 9 shows itscrystallographic map. All samples are located on a straight line. FIG.10 shows the results of pH titration. The content of soluble baseincreases slightly with the Li:M ratio. However, the total amount ofsoluble base is small. The soluble base probably originates from thesurface basicity (ion exchange) but not from the dissolution of Li₂CO₃impurity as observed in Comparative Example 1. This experiment clearlyshows that the cathode material is in the Li stoichiometric range andadditional Li is inserted into the crystal structure. Therefore, Li₂CO₃does not coexist as second phase and hence stoichiometric sampleswithout Li₂CO₃ impurity can be obtained even when Li₂CO₃ is used as aprecursor and the sintering is carried out in air.

TABLE 4 Crystallographic data Sample A B C D E F G Desired 0.925 0.9751.0 1.025 1.05 1.075 1.125 Li:M ratio Unit cell 34.110 Å³ 34.023 Å³33.968 Å³ 33.921 Å³ 33.882 Å³ 33.857 Å³ 33.764 Å³ volume c:a ratio1.0117 1.0119 1.0119 1.0122 1.0122 1.0123 1.0125 Cation 8.8% 6.6% 6.7%4.0% 2.1% 2.5% 1.4% mixing

Example 3 Large-Scale Sample Prepared in Air Using Li₂CO₃

Approx. 5 kg of LiMO₂ was prepared in one batch. Precursors were Li₂CO₃and a mixed hydroxide MOOH withM=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2). The preparation processinvolved 3 cooking steps. By heating to 700° C., a precursor with a Li:Mratio of approx. 1:1 was prepared. The furnace was a chamber furnace ofabout 20 liter volume; the sample was located in a tray ofhigh-temperature steel. This precursor was sintered at 900° C. for 10hours. During the sintering, air was pumped into the furnace. More than10 m³ of air was fed into the oven during sintering for 10 hours. Afterthe sintering, the unit cell constant was obtained by X-ray analysis,and the unit cell volume was compared with the target value. The targetvalue was the unit cell volume of the sample in Example 2 which had thebest electrochemical properties. pH titration of the sintered sampleshowed a profile very similar to that of Sample (E) in Example 2, whichproves that the 5 kg sample was basically free of Li₂CO₃ impurity. Asmall amount of Li₂CO₃ was added to ensure that the targeted unit cellvolume is achieved after the final sintering. The final cooking wasperformed in air at 900° C.

By ICP analysis, it was confirmed that the final stoichiometric ratio ofLi and M was very near to 1.00. The unit cell volume was within thetargeted region. FIG. 11 discloses the SEM micrographs of the obtainedcathode material and FIG. 12 shows a Rietveld refinement. Referring tothese drawings, the sample has a high crystallinity and is well layered.pH titration confirmed that Li₂CO₃ impurity is not present. Less than 10ml of 0.1M HCl was needed to titrate 10 g cathode to pH below 5, whichcorresponds to a content of Li₂CO₃ impurity of about 0.2% by weight orless.

As a result, this experiment shows that large-scale samples ofstoichiometric LiMO₂ with M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2),free of Li₂CO₃ impurity, can be obtained from mixed hydroxide and Li₂CO₃by solid state reaction. It is, however, required to support gastransport by pumping normal air into the furnace.

Comparative Example 10 No Air Pumping

More than 5 kg LiMO₂ were prepared in the same manner as Example 3 withthe exception that no air was pumped into the chamber furnace duringsintering. Limited air circulation was still possible through an openingof about 10 cm diameter in the door of the furnace. After sintering, theunit cell volume was obtained by X-ray analysis. The unit cell volumewas slightly less than target, indicating that the LiMO₂ phase isslightly Li-deficient, which was confirmed by pH titration. Morespecifically, more than 50 ml of 0.1M HCl was required to titrate thesample to pH below 5, corresponding to a significant amount of Li₂CO₃impurity of about 1% by weight.

As a result, this experiment shows that the natural circulation of airis not sufficient and the absence of the artificial air flow results inan incomplete reaction so that unreacted Li₂CO₃ may remain asimpurities.

Example 4 Heat Exchanger

Pumping air into a large-scale reactor at T=800˜900° C. consumessignificant additional energy in the case where the hot air is releasedto the environment after reaction. Air flow of at least 2 m³, preferablyat least 10 m³ per kg of sample is required. 2 m³ corresponds to about1.5 kg at 25° C. The heat capacity of air is about 1 kJ/kg° K and thetemperature difference is about 800K. Thus, at least about 0.33 kWh isrequired per kg of the final sample for air heating. Where the air flowis 10 m³, about 2 kWh is then necessary. Thus, the typical additionalenergy cost amounts to about 2˜10% of the total cathode sales price. Theadditional energy cost can be significantly lowered where theair-exchange is made by using a heat exchanger. The use of a heatexchanger also reduces the temperature gradient in the reactor. Tofurther decrease the temperature gradient, it is recommended to provideseveral air flows into the reactor simultaneously.

Example 5 Coin Cell Testing of Large-Scale Sample

Stoichiometric LiMO2 with M=Ni4/15(Mn1/2Ni1/2)8/15Co0.2, basically freeof Li₂CO₃ impurity, such as LiMO₂ disclosed in Example 3, waselectrochemically tested in the form of coin cell in which Li metal wasused as an anode. Cycling was performed between 3 and 4.3 V, mainly withC/5 charge and C/5 discharge rate (1 C=150 mA/g) at 25° C. and 60° C. Incomparison with the high-Ni LiNiO₂ cathode materials of Comparative.Example 7, a further improved cycling stability was observed. Thecrystallographic density of the Ni-based LiMO₂ was 4.74 g/cm³ (LiCoO₂:5.05 g/cm³). The discharge capacity was more than 170 mAh/g at C/20(LiCoO₂: 157 mAh/g). As a result, the volumetric capacity exceeds thatof LiCoO₂. This is a further significant improvement over the low-Nicathode materials of Comparative Example 8.

In Table 4 below, the obtained electrochemical results are summarized.In FIG. 13, the obtained voltage profile, discharge curves and cyclingstability are disclosed.

TABLE 4 Electrochemical properties of LiNiO₂ Capacity retention(extrapolated) after 1^(st) charge 100 cycles C/5-C/5 capacity dischargecapacity cycling, 3.0-4.3 V 3.0-4.3 V, 25° C., 25° C., 60° C., 25° C.60° C. C/10 C/1 C/20 C/20 >96% >90% >190 152 173 185 mAh/g mA/g mAh/gmAh/g

Example 6 DSC of Large-Scale Sample

Safety properties of stoichiometric LiMO2 withM=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2), free of Li₂CO₃ impuritysuch as LiMO₂ disclosed in Example 3, were tested by DSC measurement.The result is disclosed in FIG. 14. Referring to FIG. 14, the total heatcapacity is low, and the temperature where an exothermic reaction startsis high. Therefore, the safety is much improved when compared with thehigh-Ni cathode materials of Comparative Example 6.

Example 7 Polymer Cell of Large-Scale Sample

Stoichiometric LiMO₂ with M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2),free of Li₂CO₃ impurities as in Example 3 was electrochemically testedin a pilot plant polymer cell of 383562 type. The cathode was mixed with17% LiCoO₂ and the cathode slurry was NMP/PVDF-based slurry. Noadditives for the purpose of preventing gelation, i.e., preventing theincrease of viscosity, were added. During preparation such as coating,no gelation was observed. The anode was MCMB. The electrolyte was astandard commercial electrolyte free of additives known to reduceexcessive swelling.

FIG. 15 shows the cycling stability (0.8 C charge, 1 C discharge, 3˜4 V,2 V) at 25° C.—An exceptional cycling stability (91% at C/1 rate after300 cycles) was achieved at room temperature. The build-up of impedancewas low. The cycling stability exceeds that of a similar LiCoO₂ cell.This can be explained by the comparable, large irreversible capacity ofthe high-Ni LiNiO2, additionally supplying lithium which is consumedduring cycling at the anode SEI.

Also, the gas evolution during storage was measured. During a 4 h -90°C. fully charged (4.2 V) storage, a very small amount of gas was evolvedand, as shown in FIG. 16, only a small increase of thickness wasobserved. The increase of thickness was within or less than the valueexpected for good LiCoO₂ cathodes tested in similar cells under similarconditions.

This experiment provides very satisfying results regarding stability andstorage properties of LiMO2 with M=Ni4/15(Mn1/2Ni1/2)8/15Co0.2 to makethe cathode material fully competitive to LiCoO₂.

Example 8 Air Stability of Large-Scale Sample

The air stability of stoichiometric nickel-based LiMO2 withM=Ni4/15(Mn1/2Ni1/2)8/15Co0.2, free of Li2CO3 impurity of Example 3, wastested and also compared with that of the high nickel LiMO2.

Three pH titration measurements were performed. In the firstmeasurement, the content of soluble base in a fresh sample was measured.In the second and third measurements, the soluble base content of storedsamples was measured. The stored samples were held for 17 hours or 3days at 60° C. in a humidity cell containing air rich in hydrocarbons.FIG. 17 shows the results thus obtained. LiMO2 withM=Ni4/15(Mn1/2Ni1/2)8/15Co0.2 (Example 7, FIG. 17) and the high-Nisample (Comparative Example 4, FIG. 6) were simultaneously stored in thesame humidity chamber.

Referring to FIG. 17, LiMO2 with M=Ni4/15(Mn1/2Ni1/2)8/15Co0.2 is morestable in air. The decomposition kinetics is about 5 times slower thanthe decomposition of the high-Ni sample. In this connection, a carefulX-ray investigation showed clear diffraction peaks of Li2CO3 impurity inthe case of the high Ni sample, whereas no Li₂CO₃ diffraction peaks wereobserved in the case of LiMO₂ with M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2).

Example 9 Inexpensive Transition Metal Precursors

The mixed hydroxide used in Example 3 had a high tap density (>2.0g/cm3), prepared by coprecipitation of MSO4 and NaOH in the presence ofexcess ammonia (complexing additive). Ammonia in waste water causesenvironmental problems and hence is strictly regulated. Therefore, useof ammonia to reduce process costs should be avoided. It is, however,not possible to prepare the mixed hydroxide of a high density by a lessexpensive ammonia-free process.

More than 1 kg of the mixed MOOH, M=Ni4/15(Mn1/2Ni1/2)8/15Co0.2, wasprepared by an ammonia-free coprecipitation of MSO4 and NaOH at 80° C.under pH-controlled conditions. A mixed hydroxide with a narrow particlesize distribution was achieved. The tap density of the obtainedhydroxide was approx. 1.2 g/cm3. Such hydroxides prepared by a lessexpensive process can be applied where the LiMO₂ tolerates morestringent sintering conditions, i.e., it has high sintering stability.

MOOH prepared by the ammonia-free process was used as a precursor toprepare 1 kg of LiMO2 by two-step cooking, and the sintering temperaturewas 930° C. The preparation process was performed in air and the lithiumsource was Li2CO3. FIG. 18 shows the precursor hydroxide and the finalLiMO2 sample. Due to the stringent sintering, a grinding step wasrequired such that some of the particles were broken, but the particlesdid not disintegrate as was observed for high Ni-LiMO2 (example 2).Properties (press density, amount of soluble base) of the resultingpowder were tested. Electrochemical properties were tested on coin cells(Li anode) using 25° C. and 60° C. cycling. The properties were verysimilar to those disclosed in Example 5.

This experiment shows that stoichiometric LiMO2 can be successfullyprepared by an inexpensive process based upon air and Li2CO3 and also amixed hydroxide of low cost having a lower density, due to the goodsintering stability thereof.

Example 10 Reproducibility of pH Titration

5 g of a commercial stoichiometric high-Ni LiMO₂ (M=Ni_(0.8)Co_(0.2))(‘sample A’) and 5 g of stoichiometric LiMO₂ withM=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2) (‘sample B’) of Example 3were tested by pH titration. The procedure was similar to that alreadydescribed. First, 100 ml of solution was obtained by repeated soakingand decanting. Then, the pH profile of the solution was monitored bytitrating 0.1M HCl until the pH reached below 3. The amounts of 0.1M HClused to achieve pH=5 were 23 ml and 3 ml, respectively. Double theamount of HCl will be needed to reach pH 5 in the case of titrating 200ml solution obtained from 10 g cathode. In this experiment, pH titrationresults are expressed as the amount (ml) of HCl needed to titrate 10 gcathode, 46 and 6 ml/10 g, respectively. The results were wellreproducible. The same quantities were obtained for similar experimentsin which the cathode powder was soaked for a longer time during therepeated decanting of the solution.

It is important to know how much soluble base is present in thesolution, as measured by pH titration of the remaining powder. Thecathode powder, after the solution was separated, was immersed in 100 mlwater, and the pH profile of the resulting slurry was obtained byaddition of 0.1M HCl. About 20 ml of 0.1M HCl was used per 10 g ofcathode in the case of sample A so as to achieve pH below 5. On theother hand, less than 1 ml (per 10 g) was needed for sample (B). While,in the case of sample A, about 67% of soluble base was present in thesolution, in the case of sample B, >80% of the soluble base was presentin the solution.

It was also investigated whether or not the result depends on thetitration speed. If HCl is added extremely slowly (i.e., >5 hours wastaken for titration), then deviations of the pH profile would occurmostly at pH below 5. These deviations may be attributed to a slowion-exchange process (H+ in solution← →Li+ in solid). However, thisprocess is generally negligible at normal speed (i.e., about 30 min).The experiment showed that titration of the dissoluble base isreproducible. The base dissolves easily during repeated decanting. As aresult, essentially all soluble bases are present in the solution,especially if the total content of soluble bases is not too high.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A lithium ion secondary battery comprising: (a) an anode; (b) anelectrolyte; and (c) a cathode, said cathode comprising a lithiumtransition metal oxide cathode active material of the general formulaLi_(x)((Ni_(1−a−b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b))_(1−k)A_(k))_(2−x)O₂,wherein 0.65≦a+b≦0.85, 0.1≦b≦0.4, A is a dopant, 0≦k<0.05, and0.95≦x≦1.05, said lithium transition metal oxide being substantiallyfree of soluble bases such that less than 10 ml of 0.1M HCl is necessaryto titrate 200 ml of a solution containing substantially all of thesoluble bases present in 10 g of said lithium transition metal oxide toa pH less than 5, said solution being prepared by repeated soaking anddecanting of said lithium transition metal oxide.
 2. The battery ofclaim 1, wherein said battery has a discharge capacity greater than 170mAh/g at C/20 where C=150 mA/g of said lithium transition metal oxidecathode active material.
 3. The battery of claim 1, wherein said batterymaintains at least about 90% of an initial charge capacity after about300 cycles between 0.8C charge to between 3 and 4 V and 1 C discharge to2 V, where C=150 mA/g of said lithium transition metal oxide cathodeactive material.
 4. The battery of claim 1, wherein when said battery isfully charged to about 4.2 V and stored for 4 hours at 90° C., saidbattery exhibits an increase in thickness of less than about 3%.
 5. Thebattery of claim 1, wherein said cathode active material has acrystallographic density of about 4.74 g/cm³.
 6. The battery of claim 1,wherein said cathode active material has a stable layered crystallinemicrostructure having a normalized c:a ratio from about 1.0117 to about1.0123.
 7. The battery of claim 1, wherein said cathode active materialhas a stable layered crystalline microstructure exhibiting cation mixingfrom about 3.9% to about 4.5% as measured by Reitveld refinement.
 8. Thebattery of 1, wherein said cathode active material has a Li₂CO₃ impuritycontent of about 0.2% by weight or less.
 9. The battery of 1, whereinsaid cathode active material has a BET surface area between about 0.4m²/g to about 0.8 m²/g.
 10. The battery of claim 1, wherein the anodecomprises mesocarbon microbeads (MCMB).
 11. The battery of claim 1,wherein said lithium transition metal oxide is characterized by theformula of about LiNi_(4/15) (Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)O₂ orLiNi_(0.267)(Mn_(1/2)Ni_(1/2))_(0.53)Co_(0.2)O₂ or LiNi_(8/15) Mn_(4/15)Co_(3/15)O₂.
 12. A lithium ion secondary battery comprising: (a) ananode; (b) an electrolyte; and (c) a cathode, said cathode comprising astorage stable lithium transition metal oxide of the general formulaLi_(x)((Ni_(1−a−b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b))_(1−k)A_(k))_(2−x)O₂,wherein 0.65≦a+b≦0.85, 0.1≦b≦0.4, A is a dopant, 0≦k<0.05, and0.95≦x≦1.05, wherein said lithium transition metal oxide has a stablelayered crystalline microstructure having a normalized c:a ratio fromabout 1.0117 to about 1.0123, said microstructure exhibiting cationmixing from about 3.9% to about 4.5% as measured by Reitveld refinement.13. The battery of claim 12, wherein said battery maintains at leastabout 90% of an initial charge capacity after about 300 cycles between0.8C charge to 3-4 V and 1 C discharge to 2 V, where C=150 mA/g of saidlithium transition metal oxide cathode active material.
 14. The batteryof claim 12, wherein when said battery is fully charged to about 4.2 Vand stored for 4 hours at 90° C., said battery exhibits an increase inthickness of less than about 3%.
 15. The battery of claim 12, whereinsaid battery has a capacity retention of greater than about 96% after100 cycles at 25° C.
 16. The battery of claim 12, wherein said batteryhas a capacity retention of greater than 90% after 100 cycles at 60° C.17. The battery of claim 12, wherein said lithium transition metal oxideis characterized by the formula of aboutLiNi_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)O₂ orLiNi_(0.267)(Mn_(1/2)Ni_(1/2))_(0.53)Co_(0.2)O₂ or LiNi_(8/15) Mn_(4/15)Co_(3/15)O₂.
 18. A lithium ion battery comprising: (a) an anode; (b) anelectrolyte; and (c) a cathode, said cathode comprising a storage stablecathode active material of the general formulaLi_(x)((Ni_(1−a−b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b))_(1−k)A_(k))_(2−x)O₂,wherein 0.65≦a+b≦0.85, 0.1≦b≦0.4, A is a dopant, 0≦k<0.05, and0.95≦x≦1.05, said cathode active material having a content of Li₂CO₃impurity of about 0.2% by weight or less and a stable layeredcrystalline microstructure, said microstructure having a normalized c:aratio from about 1.0117 to about 1.0123.
 19. The battery of claim 18,wherein said battery has a first charge capacity greater than about 190mAh/g of said lithium transition metal oxide cathode active material.20. The battery of claim 18, wherein said battery has a dischargecapacity between about 152 mAh/g and about 173 mAh/g of said lithiumtransition metal oxide cathode active material.